Water Quality Trading in the Lower Delaware River Basin: A ...

Figure 3-1. Drainage area and point source density of watersheds with operating trading
systems and TMDL’s that have been developed in the Lower Delaware River region.
Km 2 /permit
100000
Km 2
10000
1000
100
10
1
0.1
10000
1000
100
10
1
Drainage Area, km 2
US Operating
Trading Systems
Point Source Density; Km 2 /permit
US Operating
Trading Systems
Reduction allocation strategies
Median = 2163
Mean = 8159
N = 14
Median = 41
Mean = 288
N = 71
Mean = 276
N = 12
Median = 190
Median = 12
Mean = 35
N = 32
Lower Delaware
TMDL's
Lower Delaware
Riverine TMDL's
On average, the TMDL’s in the Lower Delaware River Basin indicate that a 58%
reduction in the current input of pollutants is needed to achieve water quality goals
(Figure 3-2). How these reductions are distributed between polluters is critical to the
achieving water quality goals in an equitable, legally defensible, and economically
efficient manner. Unfortunately, far less than half of the 130 regional TMDL’s reviewed
in this study explicitly stated how the reductions are distributed among dischargers. Less
that 10% of the TMDL’s describe the details of the allocation strategy used to distribute
the reductions.
For the TMDL’s that do state how the reductions are allocated between
dischargers, a size based equal marginal percent removal (EMPR) allocation strategy is
the most commonly used method. In this strategy, all dischargers modeled in the TMDL
are reduced by equal percentages and the water quality model is run in a reiterative
fashion until the WQS are met. Thus the marginal rate of removal for all affected
dischargers is the same but the absolute quantity removed depends on the size of the
discharge. Site specific allocations of NPS reductions are not implicitly considered in the
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